It's just good practice to be understanding and accommodating to members when it comes to what they need to clear or any difficulties they have, voice chat unrelated.
Whether or not an FC uses it (and how) is up to individual culture and management. I've been in FCs where if you didn't use it, good luck breaking into a clique. And another where the regulars tended to use it for themselves but would respond to me when I typed in FC chat, either using voice or typing themselves, just fine most of the time. Back then I was pretty shy about speaking.
Restricting FC members either way seems too dogmatic for a video game, like the FCs that say you can't say "shit" in chat in a game with everything from murder to prostitution--and not because someone's kids are usually listening, either.